Ab initio study of vibrational anharmonic coupling effects in oligo(para-phenylenes)

Abstract

In this work, we clarify the nature of a previously not precisely identified Fermi dyad in the frequency range around 1600 cm−1 in oligo( para-phenylenes). To this end, we deploy a novel method to calculate third order anharmonic coupling effects in molecules. This Fermi dyad is shown to yield important information on the structural properties of the investigated materials. The nature of all vibrations contributing to this quantum mechanical resonance phenomenon is explained on the basis of a detailed normal coordinate analysis. The anharmonic coupling is then closely investigated by applying our theoretical model. In particular, we discuss the intensity redistribution among the two components of the Fermi doublet as well as their energetic separation. Subsequently, we establish a relation between these features and the structural conformation of the molecules. We show, how oligomer length and planarity of these systems can be determined from experimental Raman spectra by extracting the positions and relative intensities of the two components of the Fermi doublet. Furthermore, this Fermi resonance is shown to be sensitive to chemical modification on the molecules such as deuteration or substitution. Finally, we extend our model to electronically excited states in this class of molecules, as well as to charged species.

Raman modes in oligophenyls under hydrostatic pressure

“…The 1610 -1600 doublet in all three oligophenyls is attributed to a Fermi dyad [11,18]. A combination band that arises from two B 1 modes at ~610 cm -1 and ~990 cm -1 results in a mode of A symmetry that overlaps the fundamental aromatic C-C on-ring stretch modes of A symmetry, resulting in mixed modes of frequency ω + and ω − at ~1600 and 1610 cm -1 .…”

“…Thus, the inter-ring angle decreases with increasing oligomer length. Spectroscopic indicators of the degree of non-planarity are found in the intensity ratios and frequency separations of the 1280 cm -1 to the 1220 cm -1 modes [14] and the 1610 cm -1 to the 1600 cm -1 modes [11]. The logarithm of the 1280 cm -1 to the 1220 cm -1 intensity ratio has been found to be inversely proportional to the number of π-conjugated phenyl rings in the polymer chain [15][16][17].…”

“…The Raman spectrum of oligophenyls has been described by several authors [6 -12] and is mainly characterized by four intense modes of A g symmetry for infinite chain molecules. Four modes are investigated in this paper: the C-H in-plane bend near 1220 cm -1 , the inter-ring stretch near 1280 cm -1 , and the doublet arising from the aromatic ring stretch near 1600 cm -1 , and a combination mode of the 610 cm -1 and the 990 cm -1 modes near 1610 cm -1 [11]. In the oligophenyls, the hydrogens in the ortho-positions on adjacent rings repel, causing the phenyl rings to twist.…”

Effects of Spatial Constraints and Brønsted Acid Site Locations on para‐Terphenyl Ionization and Charge Transfer in Zeolites

“…[29] The intensity of the prominent Raman band at 1274 cm À1 that is associated with the concentration of PP 3 disappeared completely after one month. , respectively, with relative intensity change (Figure 6 A).…”

Chain length effects of p-oligophenyls with comparison of benzene by Raman scattering

“…A great deal of work has focused on these two peaks as their prominent features are related to the oligomer length and planarity of the molecules. 35,39,45 Previous works on benzene 27,29,46 observed the 1600 cm 1 doublet. The benzene line doublet is suggested to result from the "resonance splitting" to a fundamental band (low energy peak) with a frequency close to 1596 cm 1 and a combination tone (high-energy peak) from the E 2g fundamental ν 1 at 992 cm 1 and the E 2g fundamental ν 6 at 606 cm 1 .…”